JsonRestStore configurable Accept header

Description

When calling get() or query() on a JsonRestStore?, an Accept header with value "application/javascript, application/json" is added to the request.

However, with put() or add(), no such header is added. An add() without an identifier will trigger a POST request to the server, and many REST implementations will return the created entity's representation in the response body. Without the Accept header, it's impossible to tell the server that we want the response body to be JSON.